Experimental Study on the Role of Hydrogen in the Breakdown of Low-Temperature Si Epitaxy
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Experimental Study on the Role of Hydrogen in the Breakdown of Low-Temperature Si Epitaxy J. Platen-Schwarzkopf*, W. Bohne1, W. Fuhs, K. Lips, J. Röhrich1, B. Selle, and I. Sieber Hahn-Meitner-Institut, Silizium-Photovoltaik, Kekuléstraße 5, 12489 Berlin, Germany 1 Hahn-Meitner-Institut, Ionenstrahl-Labor, Glienicker Straße 100, 14109 Berlin, Germany * Email: [email protected] ABSTRACT Homoepitaxial Si layers were grown on Si(100) at temperatures of 325 - 500 °C by Electron-Cyclotron Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR PECVD) from a gas mixture of SiH4, H2 and Ar. Ar was added in order to realize high growth rates where the breakdown of epitaxy was well observed. Si disorder depth profiles derived from RBS channeling spectra were compared with hydrogen depth distributions measured by Heavy-Ion Elastic Recoil Detection Analysis (HI-ERDA) and Secondary Ion Mass Spectroscopy (SIMS). The results suggest that the transition from epitaxial to amorphous growth proceeds through two stages: (1) a highly defective but still ordered growth with the defect density increasing as the growth proceeds and (2) the formation of conically shaped precipitates of amorphous Si. Both regions act as an increasingly effective sink for excessive hydrogen which diffuses from the growth surface into the bulk of the sample. In perfectly grown epitaxial films, where the overall concentration of excessive hydrogen was low, the hydrogen diffusion tail was found to extend far beyond the interface into the Si substrate. INTRODUCTION For many years, low-temperature homoepitaxial growth of Si on Si(100) has been subject of high scientific and technological interest. Ion-assisted techniques such as ElectronCyclotron Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR PECVD) and ionassisted deposition (IAD) have been of particular interest since they allow the incorporation of additional energy into the growing surface [1,2]. For example, Si homoepitaxy by ECR PECVD has been studied with the intention to produce high-quality Si films for photovoltaic solar cells by quasi-epitaxial growth starting from a well crystallized seed layer on glass [1,3]. Ion-assisted CVD techniques are characterized by a high partial pressure of hydrogen and growth rates up to 60 nm/min [2]. It has been argued that the breakdown of lowtemperature epitaxial growth is controlled by an extremely complex interplay between beneficial and detrimental effects of the hydrogen flux to the growing surface [4]. In this paper, we present an experimental study on the breakdown of epitaxial growth mode by the formation of amorphous cones, which seems to be characteristic for ECR PECVD [3]. In particular, using MeV ion-beam scattering experiments we have correlated depth profiles of Si disorder and hydrogen concentration in order to investigate the role of hydrogen in Si epitaxy. In accordance with earlier conclusions of Varhue et al. [5] we will show that our results can be discussed in terms of models which were developed for MBE [6,7,8], where the hydrogen backg
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